Sexual selection

Sexual selection comes under 2 main classifications:

  1.  Competion can select for males with better fighting        abilities, increased body size and weapons. Deers        are an example with their large antlers for fighting –        an example of sexual dimorphism

  2.  Females making the choice as to which male they        want as a mate

If a change in trait increase reproductive success by improving chances of getting a mate then this change will be selected for, even if it decreases the overall probability of survival; it's all about getting the best genes possible into the next generation. This is why males take risks in mating and why juvenile males develop secondary sexual characteristics after maturation, to ensure that they don't suffer the costs of reproduction before they are ready for it.